RESOLVING THE LEDGER
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Add as preferred source Locals and officials in Colombia have begun a massive search and rescue effort to find survivors of a powerful earthquake that struck near San José del Palmar in Chocó, with major damage observed in the cities of Cali, Manizales and Pereira.
At least 132 people have been confirmed dead , with that number expected to rise significantly in the coming hours and days. Thousands of homes have also reportedly collapsed or been damaged.
The United States Geological Survey said the magnitude 7.4 earthquake occurred primarily due to strike-slip faulting at a depth of approximately 110 kilometers (68 miles). In simple terms, a strike-slip earthquake occurs when two blocks of Earth's crust slide horizontally past one another, rather than moving up or down.
This comes just a month or so after earthquakes in Venezuela , which may have destroyed or damaged more than 58,000 buildings and took the lives of thousands.
It's far too early to tell whether the earthquake in Colombia is connected to the one in Venezuela. But there's some evidence from Australia that nearby earthquake faults can "communicate" with one another. A seismic slip on one fault can effectively transfer stress to another nearby fault, causing an earthquake.
I've spent years experimentally testing reinforced concrete structures, some specifically designed to Colombian construction practices. My research suggests we shouldn't be surprised that so many buildings collapsed in this disaster—and that we should be concerned about the many other regions around the world with similar practices.
In buildings, slabs, beams and columns provide resistance to gravity loads. But reinforced concrete walls are needed to resist lateral loads from wind and, in rare but extreme cases, earthquakes.
But in Colombia, many buildings have thin concrete walls, typically 7 to 10 centimeters (2.8 to 3.9 inches) thick. They have only a single layer of steel reinforcement—commonly electrowelded wire mesh, which is very brittle and ruptures quickly.
The thinness of the walls also means that "confinement," where hooped steel is placed at the edges of a wall to help prevent concrete crushing, is nearly impossible to place during construction. Nor is it required in building standards.
Along with colleagues from three Colombian universities, I ran full-scale tests at the Earthquake Engineering and Structural Dynamics Laboratory at the Federal School of Technology in Lausanne, Switzerland, on concrete walls built to match common Colombian construction practice. We wanted to find out how these walls perform when shaken the way an earthquake shakes a building.
The results were concerning even before Monday's earthquake.
When an earthquake shakes a building, the ideal situation is for many small cracks to form at the base of a steel-reinforced wall. This distributes the stress.
If a single, large crack forms because of the low amount of steel, the stress is concentrated. This causes the reinforcing bars to rupture and the wall to fail.
But the walls in these buildings are so thin that, even if there were enough steel reinforcement, they would still be liable to fail during an earthquake. Our experimental tests showed there's just not enough strength for these walls to survive significant ground shaking.
This unfortunately doesn't appear to be a rare or isolated design choice—it's standard practice across the Latin American region.
A 2018 survey of nine buildings in Cali, one of the cities hit by Monday's earthquake, found that 90% had walls with a single layer of reinforcement, and 30% had walls 100 mm thick or thinner.
In 2011, a magnitude 6.3 earthquake caused widespread damage to the city of Christchurch, taking the lives of 185 people .
Most reinforced concrete buildings designed after the 1980s performed well . This was owing to the design principles introduced at the time , which required engineers to carefully consider the placement of steel to allow the structures, including the walls, to deform and resist earthquake actions.
However, some of the collapses observed were in buildings designed before these principles were introduced. One such example is the Pyne Gould building. Its collapse was likely the result of a low amount of steel used in the thin concrete core walls, where a single crack is thought to have formed, concentrating stress.
Until recently, this type of design practice—thin walls with typically a light amount of reinforcing steel—was also allowed under the Australian Standards for Concrete Structures.
After the Christchurch earthquake, this design practice became a concern for the structural and earthquake engineering community, and the Australian standards were revised in 2018 to help prevent this type of structural failure in future designs.
However, a proliferation of lightly reinforced, thin concrete walls likely exists in the building stock of Australian cities built before this revision. Testing on walls built to this same practice, including some conducted here in Australia , has shown they perform just as poorly as their Colombian counterparts under earthquake loading.
This is concerning, as Australia is not immune to strong earthquakes .
Ryan Hoult et al, Seismic performance of slender RC U-shaped walls with a single-layer of reinforcement, Engineering Structures (2020). DOI: 10.1016/j.engstruct.2020.111257
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